In recent years, the smart grid technology has been attracting attention. The smart grid concept is for connecting companies supplying energy such as electricity, gas, and water, with meters deployed in general households, by a power distribution grid provided with communication channels, and collecting and controlling data. The object of this concept is to save power, reduce costs, and to improve the reliability of information collection.
In each meter, a device is mounted, which has a sensor function and a wireless communication function, and data collection and control are performed for each smart utility network (SUN) in a certain range. The device mounted in the meter is driven by a battery (storage cell) so as to be operable even when a disaster occurs. As part of an attempt to reduce costs, the target battery life is ten years.
SUN constructs a communication system according to multihop communication (ad hoc communication) or public network communication (3G or WiMAX communication). Multihop communication is a communication method by which terminals perform relay with each other and a single master unit controls a plurality of slave units to construct a network.
In multihop communication, the data owned by the slave units is sequentially passed from the end terminal devices of the network to the master unit. The data that is passed is accumulated as the data approaches the master unit. As to the data processing amount, the processing amount is minimal at a slave unit at the end of the network, and the processing amount is maximum at the master unit. Comparing the consumption amount of batteries driving the terminals, the battery consumption of a master unit is significantly higher than that of a slave unit.
In the case of SUN, once a network is constructed, the network is maintained for a long period of time (for example, ten years), unlike the case of a multihop communication network constructed by game consoles, etc. However, if the battery of the master unit is consumed more quickly than that of the slave unit, it is not possible to maintain the network.
In order to prevent communication failures caused by the poor performance of the master unit during multihop communication, there is proposed a method of re-selecting a master unit (see, for example, Patent Document 1). By this method, a slave unit that wants to start communication searches for a master unit, and when there is no master unit nearby, the slave unit sends out test signals. The slave unit receives, from other terminals, the transfer rate and the remaining battery level, and determines a master unit. The master unit is not changed unless communication by a terminal that has become the master unit stops, or radio interruption occurs. Therefore, it is not possible to smoothen the remaining battery level among all terminals including the master unit and slave units.
In a network that is used over a long span, it is preferable that the battery consumption is smoothened among all of the terminals in the network.
Patent Document 1: Japanese Laid-Open Patent Publication No. H10-145276